Electrostatic electron lens



p 7, 1955 -K. A. HOAGLAND 2,719,243

ELECTROSTATIC ELECTRON LENS Filed July 3, 1951 4 Sheets-Sheet l Fig.

INVEN TOR. KENNETH A. HOA'GLAND ATTORNEYS p 27, 1955 K. A. HOAGLAND2,719,243

ELECTROSTATIC ELECTRON LENS Filed July 5, 1951 4 Shets-Sheet 2 2O 8O 40O 40 80 I20 JNVEN TOR. Fi 3 KENNETH AHOAGLAND BY 7 P WPMX A TTORNEYSSept. 27, 1955 K. A. HOAGLAND 2,719,243

ELECTROSTATIC ELECTRON LENS Filed July 5, 1951 4 Sheets-Sheet 3 IN VENTOR. KENNETH A. HOA GLAND I BY ATTORNEYS Sept. 27, 1955 A, HOAGLAND2,719,243

ELECTROSTATIC ELECTRON LENS Filed July 5, 1951 4 SheetsSheet 4 L I24Il/ZB IP33 7 I27 In I r 36 A fl I H? F/g 6 Fig. 7 9 8 POTENTIALINVENTOR. F g. 9 KENNETH A. HOAGLAND A TTORNEYS United States PatentELECTROSTATIC ELECTRON LENS Kenneth A. Hoagland, Newark, N. J., assignorto- Allen B. Du Mont Laboratories, Inc., Clifton, N. .l., a corporationof Delaware Application July 3, 1951, Serial No. 235,010

9 Claims. (Cl. 313-82) This invention relates to cathode ray tubes andparticularly to electrostatically focused cathode ray tubes.

Many electrode configurations have been devised heretofore to provideelectrostatic focusing for the electron beams in cathode ray or othercharged particle tubes. One example of such configurations is theso-called unipotential lens commonly used in electrostatically focusedtelevision picture tubes. This lens comprises three elements; thetubular second anode of the electron gun, usually with a limitingaperture and an outwardly flared skirt at the forward end thereof; afocusing ring electrode having the same internal diameter as the tubularportion of the second anode; and a second limiting aperture with anoutwardly flaring skirt and usually a short tubular section. Theseelectrostatic lenses are designed to focus a beam of electrons in thecathode ray tube when a voltage of approximately 20% of the anodevoltage is applied to the focus electrode. Therefore, it, is necessaryto derive a voltage of approximately 3,000 v. from some point in thetelevision power supply circuit to apply to this electrode for the usualanode voltage of about 15,000. v. Since this voltage must varyproportionately with the variations in the anode voltage, it is commonto provide a voltage divider from the anode power supply and connect atap at the desired voltage level to the focus electrode. However, sinceit is necessary to vary the voltage at the focus electrode within alimited range in order to obtain the best possible focus of the spot, ahigh voltage potentiometer must be provided which is capable ofwithstanding potential differences of 5,000 v. or more.

Tubes have also been devised heretofore in which the focusing electrodewas connected directly to the cathode of the tube. Such tubes can thenfocus only if the parts making up the electron lens have the correctsize and space relationships, and these tubes have been uneconomical toproduce in large quantities because of the many very critical tolerancesinvolved.

One of the objects of this invention is to provide an improvedelectrostatically focused cathode ray tube.

Other objects are to provide an improved electrostatic electron lens, tocombine said lens with other electrodes in an easily manufacturablecathode ray tube, and to provide an electrostatically focused cathoderay tube in which the focal properties are independent of the anodevoltage.

Still further objects will be apparent after studying the followingspecification and drawings in which:

Figure 1 shows a cathode ray tube embodying the invention.

Figure 2 is a simplified version of the tube in Figure 1. Figure 3 is agraph showing the variation of the focal strength of the electron lens.

Figures 4 and 5 show diiferent embodiments of the invention.

Figure 6 shows a typical embodiment of the electron lens.

I Figures 7 and 8 are difierent embodiments of the electron lens andFigure 9 is a graph illustrating the electrical difierences betweenFigures 7 and 8.

In Figure 1, an electron gun comprising a first grid 11, a second grid12, and an anode 13 is disposed in the neck of a cathode ray tube 14.The anode 13 in this tube may take the form of the bent anode operatingin the manner described in my copending application Serial No. 129,260,or it may take one of a number of alternative forms, as will bedescribed later. The forward end of the anode 13, that is, the endnearest the fluorescent screen or target 16 is covered by a cup-shapedcap 17 which is concave toward the forward end of the tube and containsa central aperture 18. Mounted transversely within the anode 13 is adisc 19 having a central aperture 20 which is preferably somewhatsmaller in diameter than the aperture 18.

The tubular electrode 21 coaxial with the forward portion of the anode13 is rigidly secured thereto by a mounting structure comprising aplurality of support pins 23 having their inner ends welded to theelectrodes and their outer ends held by glass rods 22 as described incopending application Serial No. 166,401 to Eric Pohle, assigned to thesame assignee. A tubular field-defining electrode 24- electricallyconnected to anode 13 is similarly secured in axial alignment with theelectrode 21 and the forward portion of the anode 13. The end of theelectrode 24 facing the anode 13 is covered by a cap or closure member27 which has a central aperture 28 and is concave toward the rear of thetube 14. The forward end of the electrode 24 is supported within theneck of the tube 14 by a radially extending member 29 with spring clips31 attached thereto to make electrical and mechanical contact with aconductive coating 32 on the inner wall of the neck of the cathode raytube 14 as described in copending application Serial No. 200,469 byHerndon W. Leighton, assigned to the same assignee. Other resilientsupports may, of course, be substituted, and they need not necessarilybe attached to electrode 24.

The electrical action of an electron gun similar to the one shown inFigure 1 will be illustrated now with reference to Figure 2. Theelectron gun shown in Figure 2 is of the socalled straight gun typeinstead of the socalled bent gun structure shown in Figure l, and itseems appropriate at this point to call attention to the two sections ofthe electrode structure as separated by the dotted line 33. To the leftof the dotted line is the electron gun structure which may be of anyknown type such as the straight electron gun shown here or the bent gunof Figure 1 or the well-known slashed-field gun, tilted gun, olf-setgun, or any combination of these guns which will produce the requiredbeam of electrons. To the right of line 33 is the electrostatic electronlens which forms the subject of this application and may be constructedin any of several forms, examples of which will be describedhereinafter. Each of the examples to be described will consist of ananode similar to anode 13 in Figure 1, a focusing electrode 21 having alarger internal diameter than the external diameter of the end of theanode, and a field-defining electrode such as electrode 24 in Figure 1.The anode and the field-defining electrode are electrically connectedtogether and the focusing electrode is directly connected to theconstant potential source such as the cathode of the tube. For thepurpose of this invention, the voltage on the cathode will be assumed tobe constant although in actual practice the video signal may beconnected thereto. The complete cathode ray tube may be made up with anyof the variations of the electron gun in combination with any of theforms of electrostatic electron lens.

The cathode 34, coaxially mounted within the grid 11, emits a stream ofelectrons 36 when heated by the heater 37. This stream is prefocused inthe manner described in the above-mentioned application Serial No.129,260

to form a constricted bundle which impinges on the aperture 20 in thedisc 19. Not all of the electrons in the beam 36 will pass through thelimiting aperture 20; some will strike the periphery and may releasesecondary electrons therefrom. These secondary electrons follow thedotted curves 38 and, as may be seen, most of these secondaries strikethe periphery of the aperture 18 in the cap 17. This aperture 18 is madesomewhat larger than the aperture 20 in order that the main bundle ofelectrons will not be impeded in passing therethrough. Most of thesecondaries which escape the periphery of the aperture 18 will becaptured by the periphery of the aperture 28 in the fielddefiningelectrode 24.

In accordance with the objects of this invention, the

electron lens structure described herein reduces the number of criticaldimensional tolerances, the main ones being the diameter A of theelectrode 21 and the distance B between electrodes 13 and 24. In Figure2, the diameter A of the focusing electrode 21 is greater than thediameter of the anode 13 by an amount 2D and the electrode 21 overlapsthe electrodes 13 and 24, each by an amount C. It has been found that ifC is equal to D,

very good focal properties result although other relation- I ships maywork equally well. lieved to be that the overlap by the electrode 21excludes stray fields from the electron lens region. Lenses in which thediameters of the anode 13 and the field-defining elec- One reason forthis is betrode 24 are .500 in. and A is .625" and B is .390" have beenfound to work particularly well although these diin which F is the focalstrength, 1 the focal length, Vo the anode potential, V is the axialpotential in the lens region and varies with z, the axial distance, andV is the derivative of V with respect to z. A curve of F versus theoverlap C has been plotted in Figure 3 to show how the focal strengthvaries with different amounts of overlap. The curve 41 was obtained bycutting equal amounts off each end of the electrode 21 and makingmeasurements in an electrolytic tank. It will be seen that when theoverlap exceeds zero, which is to say as soon as there is any overlap,the curve 41 levels out, and when the overlap exceeds approximately.040", the focal strength is substantially independent of any furtheroverlap. Below zero, that is, when there is a positive axial distancebetween a plane passed through the end of the anode 21 and a planepassed through the proximal end of either electrode 13 or electrode 24(since this positive axial distance will be the same for either), curve41 starts to drop very rapidly. This is an indication of the criticaltolerance of the length of electrode 21, unless there is some overlap.

Curve 42 differs from curve 41 in that the overlap was varied only forone end of electrode 21. This curve was obtained by taking an electrode21 of fixed length and moving it axially over either electrode 13 orelectrode 24. Thus, the overlap was increased for the electrode overwhich electrode 21 was telescoped and was correspondingly decreased forthe other electrode (either 13 or 21). As was seen in curve 41, a largeoverlap made very little difference so that curve 42 essentiallyrepresents the variation of the changing overlap at one end only. Ingeneral, the shape of curve 42 is similar to the shape of curve 41, and,as is expected, the focal strength is higher for a given degree ofnegative overlap than curve 41 due to the asymmetrical form of theelectron lens. It should be emphasized that the diameter A of electrode21 and the spacing B between electrodes 13 and 24 are the principaldeterminants of the focal strength of the electron lens, but thesedimensions may be chosen so that the electron lens will focus at anygiven distance no matter what the overlap may be, whether positive ornegative. Therefore, electron lenses, and even automatically focusedlenses, may be designed in which the overlap is negative, but the pricewhich must be paid for such design is an increase in the number ofcritical tolerances of the various parts.

It will be noted that the electrode 24 is electrically connected to thewall coating 32 of the cathode ray tube, and there is, therefore,substantially no lens action at the plane through the forward end of theanode 24. This anode is necessary in order to define the field in thelens region. Prior art electron lenses have been designed in which thewall coating 32 served as part of the fielddefining structure in thelens region. Such tubes are critical as to angular displacement of theelectron gun within the neck of the tube, and it is mainly for thisreason that the lens-defining electrode 24 has been added. Since thiselectrode is quite rigidly attached to the anode 13 and the focusingelectrode 21, the components defining the electron lens are fixed intheir spaced relationships during the manufacture of the gun wheretolerances may be easily held to a reasonable amount, instead of beingdependent on the alignment of the gun within the neck of the tube whereit is extremely difficult to hold close tolerances of alignment. Theoverlap C enters into this discussion since the electrostatic field ofthe wall coating is prevented from extending into the lens region partlyby this overlap. A negative overlap, i. e. lack of any overlap and axialspace between the ends of the electrodes, permits the field of the wallcoating 32 to have some effect in the lens region in spite of thefield-defining electrode 24 unless some care is taken to keep thecoating 32 well away from the lens region. This effect may be preventedby providing long spring fingers (Figure 4) attached to the skirt 29 ofthe electrode 24 and extending forward to make contact with the end ofthe wall coating 32 which then may be terminated that much furtherforward.

The structure shown makes possible another beneficial result inpreventing electrical leakage, corona or are discharge or breakdownbetween the cylindrical electrodes through a path established on or nearthe supporting members. Referring to Figure 1, it may be seen that thesupporting rods 22 are spaced from the focusing electrode 21 and the twoparts of the anode 13 and 24 by a plurality of metal studs 23. Thesupporting metallic studs 23 making contact with the focusing electrode21 are placed well toward the center thereof. Similarly, the supportingstuds 23 contacting the portions 13 and 24 of the anode are spaced fromthe ends of the focusing electrode 21. This spacing as illustrated at Etogether with the stepped arrangement provided by the larger diameter ofthe focusing electrode 21 as compared with the diameter of the anode 13and 24 increases the clearances and prevents electrical leakage, coronaor are discharge or breakdown therebetween.

Figure 4 shows the so-called tilted ion-trap gun comprising the cathode34, a control grid 11, a second grid 112, and an anode 213. Electronsemitted by the cathode 34 pass through the aperture in the control grid11 and the second grid 112, are deflected forwardly by the asymmetricalelectric field existing between the second grid 112 and the anode 213,and are subsequently deflected by a magnetic field indicated by thedotted region 44. Since the axis of the electron gun is normally at anangle of approximately two or three degrees with respect to the axis ofthe tube 14 (this small angle is greatly enlarged here for purposes ofillustration), the deflection by the magnetic field 44 causes theelectron beam 136 to travel substantially along the axis of the tube 14.These electrons then enter the lens region to the right of line 33substantially in alignment with the axis and subsequent forces on theseelectrons by the lens structure comprising garages the focusingelectrode 21 andfield defining electrode 24 areas described inconnection with Figure 2. The small angle which the electron gun:section to" the left of line 33 makes with the axis of the lens sectionto the right of line 33' does not materially alter the'focal'properti'esof the lens and hereinafter, in the specification and claims, the termsubstantially aligned will be used to indicate structures suchas that inFigure 4"as well as" those in Figures 1 and 2.

Another modification of the invention is shown in Figure 5. The focuslens axis. preferably should coincide with the forward portion of theanode 13 when the bent gun is properly aligned; In practice, however, itsometimes occurs that the control grid 11 and second grid 12, whichusually are assembled together prior to the assembly with the anode 13,are displaced with respect. to the axis of the rear portion of the anode13, causing the beam 36 to take a difierent path through the focusinglens for each variation of the magnetic ion-trap field 144 from theassigned position. In order that the electron beam 36 may pass properlythrough the focusing lens, it may be necessary to tilt the lens. Forexample, if grid 11 and second grid 12 are displaced by about 12 mils, atilt of the entire focus lens assembly of about 1% is necessary to causethe electron beam to coincide with the axis of the focus lens aperture.Since this small angle is less than the angle of the tilted gun shown inFigure 4, the term substantially aligned applies equally to bothstructures.

Figure 6 shows another modification of the electron lens structure tothe right of line 33. Instead of a tubular field-defining electrode, aplane apertured disc 124 may be used. The focusing electrode 21 cannotoverlap the disc 124, but since this disc has a large diameter and, asin all other modifications, as illustrated in Figure 2, is directlyelectrically connected to the anode 13, the field in the lens regionwill be quite well-defined by the structure, particularly since theoverlap is retained at the anode 13 of the focusing electrode 21. Byproper design in accordance with well-known principles, the aperturedcap 117 may be omitted, leaving only the tubular portion of the anode13.

The complete focusing structure shown in Figures 1 and 2 has been founddesirable in practice although certain simplifications are possible.Figure 7 shows the same structure without the extra disc 19 and here theaperture 18 in the cap 117 is used as the limiting aperture for theelectron beam 36. The structure shown in Figure 7 does not have quite asgreat reduction of secondary electrons as the structure shown in Figures1 and 2, unless somewhat greater care is taken in construction thereof.

The construction shown in Figure 8 is similar to that shown in Figure 7except that the caps 117 and 127 are plane instead of dish-shaped.

Figure 9 shows potential variation curves for the two structures inFigures 7 and 8 in which the spacings have been adjusted so that thefocal strength will be the same. The curve 46 may be seen to have asmaller slope in the region of the aperture 18 of Figure 7 than thecorresponding slope of curve 47 in the region of the correspondingaperture 118 as indicated at the points 48 and 49 respectively. Thus,the concave construction in Figures 1, 2 and 7 appears to be beneficialin that it removes the apertures from the high gradient fields.

While the focusing electrode is shown connected to the cathode, it maybe connected to other sources of potential including those outside thetube.

This invention has been illustrated together with only a fewmodifications. Other modifications may be apparent to those skilled inthe art without departing from the scope of the invention.

What is claimed is:

1. An electron discharge device comprising a bulb having a neck and atarget opposite said neck; an elec- 6 tron gun mounted within said neck,said electron gun comprising in order along the path of the electrons,an electron emitting cathode, a control grid, a second grid, acylindrical. anode electrode having. a concave; apertured closure memberat the end distal from said cathode, a cylindrical focusing electrodesubstantially axially aligned with said anode and overlapping; saiddistal end; a cylindrical field defining electrode having the samediameter as" said anode and having a concace: apertured closure memberat the end facing said anode, and a conductive connection between saidanode and" said field defining electrode; a conductive coatingv on theinternal wall of said neck, said conductive coating overlapping saidfield defining electrode; and a conductive connection between saidconductive coating and said field defining electrode.

2. The device of claim 1' in which there is an angle between the axis ofsaid cylindrical anode and the axis of said field defining electrode.

3. The device of claim 1 in which said anode has a bend therein.

4. A cathode ray tube comprising a cathode for emitting electrons alongan electron beam path; an electron beam focusing lens structure throughwhich said path passes, said structure comprising in order a firsttubular anode electrode having a concave apertured closure member at theend remote from said cathode; a second tubular focusing electrodeoverlapping said remote end of said anode and electrically insulatedtherefrom; a tubular field defining electrode having an aperturedclosure member at the end closer to said anode, said focusing electrodealso overlapping said end of said field defining electrode; a pluralityof structures to support said anode, said focusing electrode, and saidfield defining electrode in fixed mechanical relation, each of saidstructures comprising an insulating support rod extending generallyparallel to said beam path; a first metal post having one end affixed tosaid anode and the other end aifixed to said rod; a second metal posthaving one end affixed to said focusing electrode and the other endafiixed to said post; and a third metal rod having one end afiixed tosaid field defining electrode'and the other end affixed to said rod,each of said posts being so located that the distance along said rodfrom one post to another is greater than the shortest space path betweenone of said electrodes and another; and a direct electrical connectionWithin said tube between said anode and said field defining electrode.

5. The device of claim 4 comprising the radially outwardly extendingflange at the end of said field defining electrode remote from saidanode; and spring support means attached to said flange.

6. The device of claim 4 in which said third tubular electrode has thesame diameter as said first tubular electrode.

7. The device of claim 4 in which both said closure members aredishshaped with the concave faces thereof facing each other.

8. An electron lens structure comprising a first tubular electrode; aconcave apertured closure member at one end of said electrode; a tubularfocusing electrode overlapping said end of said first electrode,insulated therefrom, and substantially aligned therewith; a thirdtubular electrode substantially aligned with said focusing electrode,insulated therefrom, and having a concave apertured closure member atthe end facing said first tubular electrode, said focusing electrodealso overlapping said third tubular electrode; and a conductiveconnection between said first tubular electrode and said third tubularelectrode.

9. The electrode structure of claim 8 in which the proximal ends of saidfirst and third tubular electrodes have the same diameter and saidsecond tubular electrode has an internal diameter greater by an amount2D than the external diameter of said proximal ends, and said focusingelectrode overlaps said first and third electrodes each by an amount Cwhere C is at least equal to D.

References Cited in the file of this patent UNITED STATES PATENTSRudenberg Oct. 27, 1936 Rudenberg Feb. 9, 1937 Keyston et al July 5,1938 Schlesinger Apr. 4, 1939 Wienecke June 20, 1939 Schlesinger Feb.20, 1940 Bowie Aug. 13, 1940 Schlesinger July 8, 1941 Rarno Mar. 24,1942 Ramo Nov. 21, 1944 Hahn Nov. 27, 1945 8 Bachman Nov. 2, 1948 GaborNov. 2, 1948 Rudenberg Nov. 23, 1948 Moss Oct. 11, 1949 Glyptis June 5,1951 Pohle et a1. July 31, 1951 Pohle et a1 July 31, 1951 Phillips etal. May 13, 1952 De Gier Nov. 4, 1952 OTHER REFERENCES Article by Bowie,Proceedings of the Institute of Radio Engineers, pages 1482-1486, vol.36, N0. 12, December 453, Fig. 2.

